In etching of the semiconductor wafer such as silicon wafer, both wet and dry processes may apply. In the wet type, a method of immersing the wafer in a mixed acid of HF/HNO3 or in an alkali solution of NaOH, KOH, or the like is generally utilized. A method of processing the wafer with the mixed acid is also partially used in a single-wafer spinning manner. Further, in the dry type, an etching method of irradiating the wafer with excited ions or radicals generated by plasma is generally used. Various kinds of gases such as CF4, etc. are used in such method. Also, although it is disclosed that the silicon wafer may be etched by a mixture gas of hydrogen fluoride gas and water vapor, which is further mixed with ozone (JP-A-06-168922), this method may be referred to as a wet method because in reality the wafer surface gets wet.
It is necessary to polish or etch a silicon wafer covered with an oxide film in manufacturing an SOI wafer and it is, in particular, desired to polish or etch the wafer with high precision (WO2005/074033).
However, it is difficult to monitor how deep the wafer is etched during the actual etching in a so-called wet state using a hydrogen fluoride solution to which an oxidant such as nitric acid or hydrogen peroxide is added. It is likewise difficult to monitor the surface being polished by a chemical polishing method or the like, thereby being practically unable to realize feedback in the middle of polishing. Here, even if it is etching with a mixture gas of hydrogen fluoride gas and water vapor, which is further mixed with ozone, the silicon wafer surface would get wet such that the etching is substantially conducted in a wet condition and, thus, it is also difficult to monitor the surface. For example, using an optical or infrared sensor or the like, it is still difficult to monitor the film thickness in the real time. Therefore, it is not easy to secure in-plane uniformity of the wafer surface. On the other hand, in the dry type, although it is rather easy to measure the film thickness, it is likely that additional apparatuses such as a vacuum chamber, a plasma generator, etc. are required in consideration of the facility configuration, thereby making the facility larger in scale.
Meanwhile, a polishing or etching amount is strongly demanded to be controlled with high precision in recent years so that it is desirable to perform feedback during polishing or etching. In etching or polishing the wafer constituted of layers made of different kinds of materials, in particular, it is difficult to secure a predetermined polishing or etching amount since polishing or etching rates may vary in respective layers. Further, in the chemical polishing utilizing variable polishing or etching rates according to different materials, overpolishing or overetching tends to occur when the rate variation exceeds the range having been estimated. Even otherwise, it is also likely to cause overpolishing or overetching since the initial setting tends to be made to avoid the insufficient polishing or etching amount.